TERTIARY MAMMAL HORIZONS. 15 



Three perissodactyl phyla occur, namely the Hyracotherii- 

 nx, Lophiodontinae and Helaletinae, whereas at the same period 

 in America we find the HyracotJicriincv, Tapiridce, and Hc/a/ctiucF. 



Without exception in the Lutetien representatives of the 

 perissodact}'l families LopJiiodoiitiiue and Hyracotlicriiiice the 

 premolars are simpler than the molars and these animals arc 

 therefore in a stage of evolution corresponding ivith that zohich 

 i^'c find in the Wind River beds. The horses so far as I can 

 judge from personal study, from Filhol's descriptions and 

 from figures, (Gervais, '59) P. Suillus, P. parvulus, P. duvalii, 

 all belong to the primitive stage, namely, premolars simpler tJian 

 molars, no mesostyle, and are therefore in a Wind River {Pro- 

 torohippiis') rather than Bridger {Orohippiis) stage of develop- 

 ment. FiLHOL ('88, p. 182) lays great emphasis upon the fact 

 that all the so-called ' Pachynolophus ' of Issel. Pepieux and 

 Lautrec have the premolars simpler than the' molars. Further- 

 more in beds of undoubted Issel, Argenton or Bnchszveiler age, 

 no complete Anehilophns types of premolars (pm = m) occur. 

 As for the oldest Artiodactylei in either country''. Cope ('82, p. 

 71) has compared Lemoine's Lophiodoehecrns peroni of the 

 Argiles-a-lignites \\\\\\ his Trigonolestes bracliystonuis, from the 

 Wind River. Among the primates the little known Heterohyiis 

 armatns Gervais, distantly resembles Alicrosyops of the Bridger 

 in its molar teeth only, the premolars being simpler than in the 

 Bridger species. 



These are significant facts. So far as they go they indicate 

 that the known beds of Lutetien formation (having a thickness of 

 45—24 metres) are by no means equivalent to the Bridger Beds 

 (having a thickness of 800 metres), as heretofore stated, but 

 they merely correspond to a section of the Lower Bridger or 

 more probably of the L'pper W ind River formation. 



It is true that in the Helalctimr, or cursorial Lophiodonts, in 

 the fauna of Egerkijigen and Lissien, namely H. cartieri, H. 

 anneetens (and perhaps Helaletes (Hyrachyus) intermedins of 

 Selles-snr-Cher), the third and fourth premolars have double 

 internal lobes like those of H. {Desmatotheriuvi) gnyotii of the 

 Bridger. But it must be remembered as regards both Eger- 



